OncoMed slammed the brakes on a Phase II study of the cancer stem cell drug tarextumab, citing worsening response rates in the drug arm for pancreatic cancer, with red flags shooting up for overall survival as well as progression-free survival rates among patients.

As a result, the biotech plans to unblind the results and then determine what future, if any, this drug may have. Tarextumab is one of the biotech's top drugs in the clinic, following closely behind lead drug demcizumab, which is partnered with Celgene in a $3.3 billion deal.

Shares of OncoMed ($OMED) plunged 43% on the news in early trading.

"The findings communicated by the DSMB suggest a low likelihood of a statistically significant benefit in overall survival in the tarextumab ALPINE pancreatic cancer trial," said OncoMed CEO Paul Hastings in a statement. "Our aim is to quickly unblind the trial and work with our clinical sites and investigators to verify, analyze, interpret, and fully understand the data, including Notch biomarker subgroup trends, and determine next steps."

The drug--partnered with GlaxoSmithKline in a $1.4 billion deal that dates back to 2007--is targeted at Notch2 and Notch3 receptors. In this study the drug was combined with Celgene's Abraxane and used to treat stage IV cancer patients. The biotech believed that down regulating the Notch pathway gives the drug anticancer stem cell benefits, with the potential to have a big impact on the tumor microenvironment.

The program was centered on a biomarker for Notch3 expression, which is elevated in a large percentage of pancreatic cancer tumors. But the company says today that the OS rate indicated no benefit despite the Notch biomarker levels.

In a recent update, OncoMed noted that the company plans to provide updated survival data for tarextumab in a few weeks, with the data coming from a separate midstage study that pairs its drug with chemotherapy.